Alleles at the agouti locus in the mouse determine whether eumelanin (black pigment) and/or pheomelanin (yellow pigment) is synthesized by the hair follicle melanocytes. Previous studies indicated that the expression of alleles at the agouti locus involve a tissue interaction mechanism between the developing dermis and epidermis. It is clear that the agouti alleles act via the dermis to specify the type and distribution of pigment synthesized by the follicular melanocytes. However, evidence has accumulated which suggests that the epidermis is not passive in this tissue interaction process, but may play an active and important role. Utilizing the techniques of dermal-epidermal recombination of embryonic skin of various ages and agouti genotypes this developmental study will: 1. Provide further evidence for the role of the epidermis in the expression of the agouti locus by determining the site of action of the intermediate yellow and sienna yellow agouti alleles as well as the agouti suppressor mutation. 2. Determine if the action of the yellow, viable yellow and mottled agouti alleles represents a temporal loss of competence by the epidermis to respond to a dermis of different agouti genotype and, if this loss of competence exists, does it represent an early reflection of a characteristic common to all agouti alleles. 3. Determine if the yellow, viable yellow and mottled agouti alleles act primarily on the dermis, as a previously proposed, or primarily on the epidermis and secondarily on the dermis. The results of this study will allow the formation of a gene action mechanism which will define the roles of the developing dermis and epidermis in the expression of the agouti locus and provide the groundwork for future studies on the tissue interaction mechanism. The action of alleles at the agouti locus illustrates the complexity of interactions between a differentiating cell, its cellular environment and the genetic make-up of both the responding and inducing tissues. A knowledge of these events is basic to our understanding of the genetic control of development. Studies such as the one proposed here are important in that they provide the groundwork that will eventually lead to an understanding and treatment of gene related development abnormalities in humans.